WordPress init hook called multiple times

I did a fresh installation of wordpress 3.0.4 and attempted to write a simple theme for just the sake of trying.

In functions.php, I created a function called my_setup and hooked it up to init. I noticed that my_setup() is called multiple times and I tried to use a global flag variable to control the execution of the function but to no avail. How do I ensure that my function is only called once?

       <?php
       // global variables
       $setup_run_before = 0;
       ?> 

      <?php

      function my_setup() {
              global $setup_run_before, $a;
              if($setup_run_before == 0) {
                      $setup_run_before = 1;
// this section is always called even with global variable.
              }       
      }       





      ?>

      <?php
      add_action('init', 'my_setup');

      ?>

Regards,
John Doe